1. Field
The aspects of the disclosed embodiments relate to the emptying of the floor of a black liquor recovery boiler, in particular, but not solely, to the emptying of smelt and/or wash water from the black liquor recovery boiler when the boiler is being shut down.
2. Brief Description of Related Developments
A black liquor recovery boiler is used for combusting black liquor generated in connection with pulp manufacture, containing various sodium salts besides organic matter and water. During the operation of the boiler, these salts form a smelt pool on the furnace floor, from which smelt continuously flows through smelt spouts to a dissolving tank. The smelt spouts are typically located approximately 250 mm above the level of the furnace floor. Typically, there is continuously a smelt layer of at least approximately 300 mm on the furnace floor during operation.
When the black liquor boiler is shut down for maintenance, for example, the furnace floor remains covered with smelt. When the floor cools down, the smelt solidifies and forms a hard “cake”, which must be removed by water washing or by chiseling if the aim is to clean the floor for maintenance work or inspections. The cleaning of the floor significantly extends the shutdown period, so a method and apparatus for removing smelt from the furnace by pumping have been devised in order to save time, as disclosed in the Finnish patent application no. 974206. The pumping is started in a situation where the surface of the smelt has reached the level of the lower edge of the smelt spout opening when the boiler is being shut down. Heating of the smelt is continued with a gas or oil flame and a spiral pump is used for the pumping.
When using a spiral pump, the smelt is pumped from the furnace floor using a rectilinear pipe having a pipe mouth at one end and a pump drive at the other end. There is a curved portion around the middle of the pipe, forming a discharge pipe through which the smelt is discharged from the pipe. In black liquor recovery boilers where the floor profile is such that the smelt pool is at its deepest close to the smelt spouts, it may not be possible to pump smelt from the deepest location of the pool with a spiral pump; rather, a considerable amount of smelt may, in many cases, remain on the furnace floor after the pumping. This extends the time needed for water washing and thus slows down the shutting down of the boiler. Another problem associated with the use of spiral pumps is that their installation requires a fairly long period of work in the immediate vicinity of the smelt spouts.
The Finnish patent application no. 20065668 discloses a method and device for emptying the floor of a black liquor recovery boiler of smelt when the boiler is being shut down. The floor is emptied by sucking smelt from the furnace with a smelt eductor. Negative pressure is generated in the device by conducting pressurized gas into a suction pipe of the eductor so that the pressurized gas is discharged in the discharge direction of smelt.